This invention relates to improved catalysts for polymerization of alpha-olefins, and more particularly, to catalysts for polymerization of alpha-olefins to products of improved morphology.
It is well known to polymerize alpha-olefins in the presence of catalysts generally comprising an organometallic promoter and a supported catalytic complex comprising an intimate association of reduced Group IVB and/or VB metal halides, divalent metal halides and one or more aluminum compounds. Such complexes typically are prepared by reaction of one or more higher valent Group IVB or VB metal compounds, support materials comprising at least one catalytically inert divalent metal compound, and organoaluminum compounds corresponding to the formula AlR.sub.n X.sub.3-n, wherein R is hydrocarbyl, X is halogen and 0&lt;n.ltoreq.3. Examples of such catalyst components are described in detail in U.S. Pat. No. 3,644,318 (Diedrich et al.), U.S. Pat. No. 3,901,863 (Berger et al.), U.S. Pat. No. 4,199,476 (Melquist et al.) and U.S. Pat. No. 4,233,182 (Hoff et al.), all of which are incorporated herein by reference.
The above-described catalysts typically exhibit sufficiently high activities in polymerization of alpha-olefins that useful products can be obtained without removal of catalyst residues. This, of course, leads to important advantages in terms of process efficiency. However, catalyst performance is not entirely satisfactory from the standpoint of polymer morphology.
While not wishing to be bound by theory, it has been speculated that the small particle size of the supported catalyst complex and/or the tendency of the complex to fragment during polymerization use, e.g., during pumping of catalyst component and/or due to the exothermic polymerization reaction itself, result in relatively high levels of small polymer particles and a relatively broad distribution of polymer particle sizes.
Whatever the cause, production of small polymer particles and polymer of broad particle size distribution are disadvantageous for several reasons. From the standpoint of polymerization process efficiency, high levels of small polymer particles can cause problems because the particles tend to accumulate in, and plug, process lines and filters. From the standpoint of handling and processing of polyolefins, small polymer particles and broad particle size distribution can be disadvantageous because polymer bulk density often is lower than desired and an extrusion and/or pelletization step often is required prior to processing.
In the past, various means for improving polymer particle size have been proposed. One approach has been to prepare supported catalyst complexes using support materials which, due to their size, shape and/or chemical composition, yield complexes that are improved in terms of polymer morphology. See, for example, U.S. Pat. No. 3,787,384 (Stevens et al.)--supported catalyst complexes prepared from silica, alumina or silica-alumina support materials having particle size ranging from 10 to 500 microns; U.S. Pat. No. 3,953,414 (Galli et al.)--spherical or spheroidal supported catalyst complex prepared from hydrated magnesium chloride which has been melted and sprayed into a current of hot nitrogen or air through nozzles having orifices of appropriate diameters; U.S. Pat. No. 4,111,835 (Foschini)--supported catalyst complexes prepared from hydrated Mg chloride in the form of spheroidal particles of 10 to 70 microns; U.S. Pat. No. 4,104,199 (Hoff)--supported catalyst complexes prepared from hydrated Mg stannate support materials; and U.S. Pat. No. 4,233,182 (Hoff et al.)--supported catalyst complexes prepared from support materials which are divalent metal salts of phosphorus acid esters.
A second approach has been to pretreat supported catalyst complexes with minor amounts of alpha-olefins to form encapsulated particles of greater size and resistance to fragmentation. See, for example, U.S. Pat. No. 4,190,614 (Ito et al.).
A third method for improving supported catalyst complexes in terms of polymer morphology involves the use of modifying compounds. Thus, U.S. Pat. No. 4,039,472 (Hoff) discloses treatment of complexes of the type described in the aforesaid U.S. Pat. Nos. 3,644,318 and 3,901,863 with anhydrous HCl to improve polymer morphology. Anhydrous HCl also can be used for purposes of temporary and reversible deactivation of supported catalytic complexes as taught in U.S. Pat. No. 4,130,699 (Hoff et al.).
Among the foregoing methods, the first is somewhat limited in terms of the number of materials that will yield the desired effect as a result of chemical composition and by the cost and complexity of obtaining support material particles of specific shapes and/or sizes. Similarly, alpha-olefin pretreatment is disadvantageous because it can add cost to the overall polymerization process. In addition, encapsulated catalyst complex particles resulting from pretreatment often are more difficult to convey and feed to a reactor than untreated particles.
The use of modifying compounds to improve supported catalyst complexes in terms of polymer morphology is potentially a simple and inexpensive method assuming the existence of effective and easy-to-use modifiers that do not adversely affect catalyst performance, e.g., activity, polymer rheology, to a substantial degree. Anhydrous HCl meets these criteria for the most part though its use is complicated somewhat because it is used as a gas, and accordingly, difficulties may be encountered in metering the precise amounts necessary to attain desirable improvements in morphology while avoiding undesirable agglomeration of particles or other undesirable effects.
From the foregoing, it can be appreciated that there remains a need for improvements in the above-described supported catalyst complexes in terms of polymer morphology. It is an object of this invention to provide such improvements. A further object is to provide supported catalyst components that are improved in terms of polymer morphology but only insubstantially affected in terms of other catalytic properties. A further object of the invention is to provide an improved alpha-olefin polymerization catalyst component and catalyst based thereon, as well as a method for production thereof and for use in polymerization of alpha-olefins. Other objects of the invention will be apparent to persons skilled in the art from the following description and the appended claims.
We have now found that the objects of this invention can be attained by modification of the above-described supported catalyst complexes with appropriate amounts of certain hydrocarbon-soluble aromatic nitro compounds. Advantageously, the modification procedure is simple and does not add substantial cost to the overall polymerization process. Further, as a result of the modification, catalyst complexes are improved in terms of polymer morphology without substantial adverse effects on other properties. In particular, particle size is increased and particle size distribution narrowed such that process efficiency is improved and polymer processing and handling are facilitated.
U.S. Pat. No. 3,377,326 (Loveless et al.) may be of interest with respect to the present invention in disclosing the use of aromatic nitro compounds in olfein polymerization, though such use is for purposes unrelated to improving morphology. Rather, the patentee discloses addition of phosphorus trihalides and oxidants, including nitroaromatics (see Column 3 lines 33-35) to olefin polymerization catalyst made up of vanadium salts and organometallic components for the purpose of increasing activity. Nitrobenzene is specifically disclosed as a suitable oxidant; Example V illustrating addition of the compound and PCl.sub.3 to an ongoing ethylenepropylene copolymerization.
In contrast to Loveless et al.'s use of nitroaromatics to increase activity of an unsupported vanadium salt-based catalyst by simple addition of the nitro compound to the remaining catalyst components or preformed catalyst, the present invention involves treatment of a supported, Group IVB and/or VB metal-based catalyst complex with aromatic nitro compound prior to polymerization and prior to formation of the ultimate catalyst. Further, as a result of the treatment according to the present invention, polymer morphology is improved but activity decreases somewhat. This is in direct contrast to Loveless et al.'s use of nitroaromatics to increase activity.
U.S. Pat. No. 4,189,557 (Klaerner et al.) also may be of interest in disclosing that morphology of polyolefins prepared using a catalyst comprising a trialkylaluminum or dialkylaluminum chloride and a component "consisting entirely or substantially of titanium trichloride" can be improved by moistening the titanium trichloride component with a hydrocarbon mixture comprising 100 parts by volume of an alkane hydrocarbon from the pentane, hexane or heptane series and 10 to 1000 parts by volume of a benzene hydrocarbon selected from benzene and certain alkylbenzenes; and subsequently drying the moistened component. However, Klaerner et al. fails to disclose or suggest either the supported catalyst complexes or aromatic nitro compounds used according to the present invention. Further, treatment with aromatic nitro compounds according to this invention involves more than the simple moistening involved in Klaerner et al.
Workers in our laboratories also have found that addition of a nitromesitylene-diethylaluminum chloride reaction product to a catalyst comprising diethylaluminum chloride and an organoaluminum reduced, electron donor-complexed, unsupported titanium trichloride component such as that disclosed in U.S. Pat. Nos. 3,984,350 (Karayannis et al.) or 4,210,738 (Hermans et al.) may result in improvements in the particle size of polypropylene. However, neither extension of such work to catalysts containing supported transition metal-containing complexes, nor treatment of such complexes with hydrocarbon-soluble nitroaromatics according to the present invention was proposed.